Microwave investigation of the drying behavior of CA-cement bonded refractory monolithics

Cement bonded refractory monolithics are typically used in the field of industrial high temperature treatment of products between 600 °C and 2000 °C. Calcium aluminate (CA) cement is used in low concentrations to obtain a primary green strength after the addition of water. The formation CA-hydrate phases ensure a safe installation. However, before the first use of these linings it is necessary to remove the hydrate bond and the residual pore water quantitatively to avoid serious damages due to an explosive expansion of water when water steam is formed. The water expulsion is a complex process consisting of different subsequent stages in the temperature range, of 250 °C to 650 °C that firstly starts with the dry-out of pore water and ends with the decomposition of the most stable hydrate phases. At present state of art refractory linings are dried during the first heat-up of the furnaces whereat the temperature increase is very shallow until 700 °C. The major drawback of the procedure is that it is based on empirical values that include a safety coefficient. In consequence the procedure is not optimized in terms of time, energy consumption and in consequence in CO
2
emission. Microwave moisture measurements could fill this gap, because microwaves are sensitive to humidity and hence this method could be used to determine, contact free, the decomposition temperatures of the hydrate phases and the total dry out of the material. In this paper we report on our first experience how microwave measurements could be applied to investigate the drying behavior that is related to the decomposition of the hydrate phases in the temperature field between 250 °C and 650 °C.